The global shortage of freshwater, especially in agriculture, has become a serious threat to food security and safety; therefore, using brackish water with soil amendments in facility-grown vegetable production has drawn more attention recently. In this study, the effects of the combined use of peat or biochar at different rates (1%, 3%, and 5%) on soil properties, plant growth, and physiological responses were investigated in cherry belle radish (Raphanus sativus L.) irrigated with brackish water (5 g/L). The results revealed that peat significantly cut soil electrical conductivity (EC) by 42.61% and the pH value by 0.29, while biochar reduced soil EC by 32.44% but increased the pH value slightly. Both peat and biochar significantly increased the chlorophyll content and photosynthetic characteristics in cherry belle radish leaves and effectively promoted the accumulation of plant biomass. The net photosynthetic rate (Pn) of cherry belle radish in the soil amended with 5% peat or with 3% biochar increased by 89.06% and 85.94%, respectively, compared with CK, while the fresh weight of fleshy roots rose by 74.40% and 50.27%, respectively. This study further found that peat and biochar had effectively inhibited lipid peroxidation and proline accumulation in plants. The plasma membrane permeability was reduced by 41.18% and 39.90%, and the malondialdehyde (MDA) content decreased by 49.66% and 41.90%, respectively. In addition, peat and biochar significantly improved ion homeostasis in plants by increasing the ratios of K+/Na+ and Ca2+/Na+ in leaves, with the increment amplitudes reaching 90.21%/81.45% and 60.47%/79.15%, respectively. Nevertheless, biochar exhibited a superior effect compared to peat on balancing plant ions. In conclusion, a proper application of peat or biochar under brackish water irrigation has a significant potential to ameliorate soil properties and alleviate salt stress in plants, providing a safe approach for using brackish water with soil amendments in facility vegetable production in arid and semi-arid regions.